Logo in expositio theme

Hi,
I’m setting up a page using the Expositio theme from wpshower. I like everything about this theme except for the fact that I can’t figure out how to insert a .png for my logo instead of the text of the webpage title. I’ve looked for help on wpshower forums, but got no replies.

I know that I must insert a line with a piece of code directing to the logo file into my header.php, but I can’t figure out where and how. The attemps I’ve made have either ended with an error or with the logo appearing underneath the menu sidebar rectangle (partially hidden).